NT-proBNP ratio is a potential predictor for COVID-19 outcomes in adult Chinese patients: a retrospective study

Despite the progressive decline in the virulence of the novel coronavirus, there has been no corresponding reduction in its associated hospital mortality. Our aim was to redefine an accurate predictor of mortality risk in COVID-19 patients, enabling effective management and resource allocation. We conducted a retrospective analysis of 2917 adult Chinese patients diagnosed with COVID-19 who were admitted to our hospital during two waves of epidemics, involving the Beta and Omicron variants. Upon admission, NT-proBNP levels were measured, and we collected demographic, clinical, and laboratory data. We introduced a new concept called the NT-proBNP ratio, which measures the NT-proBNP level relative to age-specific maximum normal values. The primary outcome was all-cause in-hospital mortality. Our analysis revealed a higher in-hospital mortality rate in 2022, as shown by the Kaplan–Meier Survival Curve. To assess the predictive value of the NT-proBNP ratio, we employed the time-dependent receiver operating characteristic (ROC) curve. Notably, the NT-proBNP ratio emerged as the strongest predictor of mortality in adult Chinese hospitalized COVID-19 patients (area under the curve, AUC = 0.826; adjusted hazard ratio [HR], 3.959; 95% confidence interval [CI] 3.001–5.221; P < 0.001). This finding consistently held true for both the 2020 and 2022 subgroups. The NT-proBNP ratio demonstrates potential predictive capability compared to several established risk factors, including NT-proBNP, hsCRP, and neutrophil-to-lymphocyte ratio, when it comes to forecasting in-hospital mortality among adult Chinese patients with COVID-19. Trial registration Clinical Trial Registration: www.clinicaltrials.gov NCT05615792.


Outcome and statistical analysis
The primary endpoint of this cohort study was in-hospital all-cause mortality.Patients were followed from admission until the observation data points (February 1, 2020, to April 26, 2020, and December 1, 2022, to February 28,  2023) to assess the risk of in-hospital mortality.Attending physicians and study physicians thoroughly evaluated the medical records to gather the necessary follow-up data.
The normality of distribution and homoscedasticity of each dataset were evaluated using D' Agostino's and Pearson's omnibus tests.For normally distributed continuous data, the mean values along with their standard deviations were presented and compared using Student's t-test.The Wilcoxon rank-sum test was employed to compare continuous non-normally distributed data, where the median values and interquartile ranges (IQR) were reported.If applicable, categorical data, expressed as counts (percentages), were compared using the Chisquare test, Fisher's exact test, and Cochran-Mantel-Haenszel test.Bonferroni's correction was used to evaluate the statistical significance of multiple comparisons.
The binary cutoff value was selected based on the maximum Youden index.Survival analysis utilized Kaplan-Meier curves and Cox proportional hazard models to compare the incidence of endpoint events among both the overall patient population and different subgroups.The time-dependent receiver operating characteristic (ROC) curve is used to explore the optimal predictor for in-hospital mortality of COVID-19.Statistical analysis and data visualization were conducted using GraphPad Prism (version 9.0; San Diego, CA, USA), R (version 4.1.1;R Foundation for Statistical Computing, Vienna, Austria), and SPSS (version 24.0; IBM, Armonk, NY, www.nature.com/scientificreports/USA).The main R packages used include "survival", "survminer", "pROC" and "timeROC".The threshold for statistical significance was set at P < 0.05.
Upon admission, the 2020 cohort exhibited higher systolic and diastolic blood pressure, respiratory rate, and pulse rate, as well as lower initial neutrophil and white blood cell counts, higher initial lymphocyte counts, and lower levels of D-dimer (Table 1).Indicators related to liver and renal function showed significant differences between the 2020 and 2022 subgroups, except for lactate dehydrogenase.The concentration of alanine aminotransferase was found to be higher in 2020 when compared to 2022.Conversely, the concentration of aspartate aminotransferase was observed to be higher in 2022 compared to 2020.However, the renal function of patients  www.nature.com/scientificreports/ in 2022 was significantly worse compared to patients in 2020.In 2022, there were noticeable differences in the patients' renal function compared to 2020.Specifically, levels of blood urea nitrogen and creatinine were higher in 2022, while estimated glomerular filtration rate (eGFR) was lower.Additionally, concentrations of D-dimer were higher in 2022 compared to 2020 (Table 1).

Clinical outcomes during hospitalization
In this study, a total of 376 individuals (12.9%) died while hospitalized, including 87 (9.9%) deaths in 2020 and 289 (14.2%) in 2022 (P = 0.002) (Fig. 1, Table 1).Cumulative survival curves indicated a higher in-hospital mortality rate among COVID-19 patients in 2022 compared to 2020 (Fig. 2).Among the entire participant group (Fig. 3) as well as the 2020/2022 subgroup (Fig. S1), the initial NT-proBNP ratio exhibited the largest area under the curve (AUC) in the time-dependent receiver operating characteristic (ROC) curve, indicating its potential predictive ability for in-hospital mortality (AUC = 0.826, P < 0.001).Additionally, hsCRP was shown to be a more reliable potential factor than NLR in 2020, whereas NLR surpassed hsCRP in predictive value in 2022 (Fig. S1).

NT-proBNP ratio risk stratification
To assess the risk stratification of in-hospital mortality among COVID-19 participants, we categorized 2917 patients into quartiles based on the ascending order of the NT-proBNP ratio (Fig. 5).Univariable and multivariable Cox regression analyses of the entire COVID-19 patient cohort revealed quartiles 2, 3, and 4 had a significantly higher risk of death compared to the first quartile (lowest) (Table 2).Using a cutoff value of 0.37, we defined the low-risk group as NT-proBNP ratio < 0.37, the medium-risk group as 0.37 < NT-proBNP ratio < 0.89, and the high-risk group as 0.89 < NT-proBNP ratio < 77.78 (Fig. 5).There were no significant differences in the cutoff value of the NT-proBNP ratio and quartiles among the total patients and the two subgroups.The value in 2022 was the same as that of the total patients, slightly higher than that of 2020.Therefore, in the 2020 group, we defined the low-risk group as NT-proBNP ratio < 0.31, the medium-risk group as 0.31 < NT-proBNP ratio < 0.38, and the high-risk group as 0.38 < NT-proBNP ratio < 77.78.In the 2022 group, we defined the low-risk group as NT-proBNP ratio < 0.37, the medium-risk group as 0.37 < NT-proBNP ratio < 1.22, and the high-risk group as 1.22 < NT-proBNP ratio < 76.61 (Fig. S5).

Discussion
This study highlights the potential of the NT-proBNP ratio as a tool in predicting in-hospital mortality among adult COVID-19 patients.The elevation of the NT-proBNP ratio upon admission may serve as an early warning sign for clinicians to identify high-risk individuals who require closer monitoring and immediate intervention.
NT-proBNP, which reflects hemodynamic stress, has shown its value in risk stratification for heart failure (HF) and other conditions like pulmonary embolism and pneumonia following SARS-CoV-2 infection 4,7 .Elevated NT-proBNP levels have been independently associated with COVID-19 mortality, even after considering factors such as chronic or acute HF 4,8 .Several studies have suggested a potential link between NT-proBNP and worse outcomes in COVID-19 patients, supporting its use for risk stratification [9][10][11] .
Our research demonstrated that the AUC NT-proBNP was smaller than that of NLR or hsCRP in all three groups.However, the NT-proBNP ratio had the highest AUC in the time-dependent ROC curve and showed the highest adjusted hazard ratio in multivariate Cox regression analysis.Although NT-proBNP is known to be influenced by renal function, and we observed worse renal function in the 2022 patient cohort, our findings demonstrate that the NT-proBNP ratio emerged as a potential factor compared to NT-proBNP/eGFR (AUC = 0.757, P < 0.001) for predicting in-hospital mortality in COVID-19 patients.
The cytokine storm, characterized by uncontrolled and exaggerated immune responses, can result in diffuse alveolar damage, multi-organ failure, and ultimately, death in COVID-19 12 .Elevated CRP levels can signify a secondary "cytokine storm" induced by secondary bacterial pneumonia, which can independently contribute to multiorgan damage 13 .Initial elevation in CRP levels has been shown to be an independent indicator of critical illness in COVID-19 patients (AUC 0.783, P < 0.001) 14 .Additionally, in a time-to-mortality Cox regression analysis, a threshold cutoff of CRP ≥ 40 mg/L demonstrated good performance in predicting COVID-19 mortality 15 .These findings highlight the significance of CRP as a biomarker in assessing disease severity and prognosis in COVID-19.Our research findings also indicated that CRP has shown promising predictive performance (cutoff value = 69.9mg/L, adjusted HR = 5.29, P < 0.001) for in-hospital mortality among all COVID-19 patients (AUC = 0.777, P < 0.001).
Researchers identified a common immune pattern in the peripheral blood of 63 hospitalized COVID-19 patients, regardless of their diverse backgrounds, showing significant changes in T cell characteristics and specific increases in cytokines/chemokines 16 .This immune signature enables early patient identification and risk-based categorization 16 .In COVID-19 patients, a higher NLR has been linked to clinical deterioration and increased mortality 17,18 .In our study, the AUC for NLR (with a cutoff value of 7.29) in predicting mortality among the total 2,917 COVID-19 patients was 0.788 (0.768 in 2020 with a cutoff value of 3.83, and 0.780 in 2022 with a   24 , and 0.9232 25 However, the number of participants in each study was significantly smaller compared to ours, with the largest study having only 1004 participants.The study with the highest enrollment size also had the highest AUC value and a cutoff value of 11.8 24 .Four studies were conducted in China, while the remaining three took place in America 20 , Turkey 22 , and Pakistan 23 .Except for one study from America involving 125 patients, which had a lower AUC than ours, the rest demonstrated higher AUC values.These variations highlight the heterogeneity in findings across different studies.Overall, our results support the importance of NLR as a potential indicator for COVID-19 mortality, with our study demonstrating favorable AUC values and cutoff points compared to previous research. In 2020, at the onset of the pandemic outbreak, there was limited understanding of the disease, leading to widespread fear and a surge of infected patients seeking hospitalization.Our results indicate a higher incidence of concomitant pathologies, such as secondary bacterial pneumonia in the 2020 group.However, in 2022, with a subsequent wave of infections, the public's response changed.The fear subsided, and people became more aware of how to handle the virus.Additionally, as the virulence of SARS-CoV-2 decreased, the majority of individuals experienced milder symptoms compared to their initial infection.As a result, except for patients with comorbidities, most individuals chose not to undergo hospital treatment.In the 2022 group, a greater proportion of patients with more comorbidities exhibited lymphocyte depletion.During the initial outbreak in 2019, a greater number of patients succumbed to SARS-CoV-2 infection at an early stage or displayed signs of interstitial pneumonia in chest computed tomography (CT) before viral nucleic acid detection.In contrast, in 2022, fewer patients died at an early stage or showed signs of chest CT abnormalities, and if abnormalities were present, they appeared after viral nucleic acid detection.In addition, due to the promotion of China's national COVID-19 vaccination policy, the entire population has been vaccinated once or more times in and after 2021 26 .All COVID-19 patients enrolled in 2022 have received the vaccination.However, in the research cohort, none of the COVID-19 patients in 2020 received the vaccination because of the lack of effective vaccines in the emergency situation.Furthermore, we observed that the concentration of D-dimer was higher in the 2022 patients compared to those in 2020.This observation is likely associated with thrombo-inflammation, a process implicated in adverse events related to COVID-19.Thromboinflammation is characterized by dysregulation of endothelial antithrombotic function in response to inflammatory stress, resulting in leukocyte recruitment, complement and platelet activation, and enhanced microvascular coagulation 27,28 .These findings highlight the evolving nature of the disease and the involvement of different pathologies and mechanisms over time.

Limitations
It is important to address the limitations of our study, which was a single-center, retrospective, observational study.The lack of consistent evaluation of NT-proBNP concentration at hospital admission in all patients in 2020 may introduce selection bias and underestimate mortality in the 2020 subgroup.In the second epidemic, patients were arranged to be hospitalized in various departments of hospitals due to medical squeeze, and doctors in different departments also had different understanding of the impact of the novel coronavirus on other systems.The detection rate of NT-proBNP is not high.However, for inclusion in the study, only those patients with detectable NT-proBNP values were enrolled.In addition, during the initial outbreak of the pandemic in 2020, there was a rapid and substantial increase in the number of patients, exceeding the capacity of laboratory to confirm SARS-CoV-2, and cases clinical diagnosed based on imaging evidence of pneumonia, along with relevant epidemiological background and clinical symptoms were also included.While our findings may not fully represent the entire COVID-19 population, they provide essential insights for prognosticating patients infected with the continuously mutating SARS-CoV-2 virus.

Conclusion
Based on our study findings, we propose that the NT-proBNP ratio serves as a potential indicator for adult COVID-19 patients.These results contribute to our understanding of risk stratification and provide valuable information for clinical decision-making in managing COVID-19 patients.However, further research and validation studies are needed to confirm and expand upon these findings.

Figure 1 .
Figure 1.The flowchart of study design.

Figure 3 .
Figure 3.The NT-proBNP ratio and all the other clinical indicators for in-hospital death of total COVID-19 patients by time-dependent receiver operating characteristic (ROC) curves.

Figure 4 .
Figure 4.The Kaplan-Meier survival curve of enrolled COVID-19 patients with high and low NT-proBNP ratio in the total participants (A), in 2020 subgroup (B) and in 2022 subgroup (C).

Figure 5 .
Figure 5. (A) The Kaplan-Meier survival curve of total COVID-19 patients with quartiles of NT-proBNP ratio.(B) Risk stratification of in-hospital mortality according to NT-proBNP ratio among total COVID-19 participants.

Table 2 .
Univariate and multivariate Cox regression of NT-proBNP ratio in the total patients and different subgroups.